I. Field of the Invention
The present invention relates to semiconductor circuits and to charge pumps used therein. More specifically, the invention relates to a simplified charge pump system for providing a voltage to various semiconductor integrated circuits or portions thereof. The invention is particularly applicable to dynamic random access memory devices (DRAMs).
II. Description of the Related Art
System designs are routinely constrained by a limited number of readily available power supply voltages (Vcc). For example, consider a portable computer system powered by a conventional battery having a limited power supply voltage. For proper operation, different components of the system, such as display, processor, and memory components employ diverse technologies which require power to be supplied at various operating voltages. Components often require operating voltages of a greater magnitude than the power supply voltage and, in other cases, a voltage of reverse polarity. The design of a system, therefore, must include power conversion circuitry to efficiently develop the required operating voltages.
One such power conversion circuit is known as a charge pump. The demand for highly-efficient and reliable charge pump circuits has increased with the increasing number of applications utilizing battery powered systems, such as notebook computers, portable telephones, security devices, battery-backed data storage devices, remote controls, instrumentation, and patient monitors, to name a few.
Inefficiencies in conventional charge pumps have led to reduced system capability and lower system performance in both battery and non-battery operated systems. Inefficiency can adversely affect system capabilities, e.g., limited battery life, excess heat generation, and high operating costs. Examples of lower system performance include low speed operation, excessive operating delays, loss of data, limited communication range, and inability to operate over wide variations in ambient conditions including ambient light level and temperature.
In addition to constraints on the number of power supply voltages available for system design, there is an increasing demand for reducing magnitudes of the power supply voltages. The demand in diverse application areas could be met with highly efficient charge pumps that operate from a supply voltage of less than five volts.
Such applications include memory systems backed by 3 volt standby supplies, processor and other integrated circuits that require either reverse polarity substrate biasing or booted voltages outside the range of 0-3 volts for improved operation.
One such known charge pump system is a two stage charge pump. Two stage charge pump systems have proven to be effective at providing semiconductor components with the necessary input voltage particularly where the system voltage is below 3 volts.
For purpose of simplification, the following discussion will focus on the charge pumps which must produce a positive voltage greater than the most positive supply voltage Vcc; however, the concepts discussed are also applicable to charge pumps designed to produce a negative voltage from a positive Vcc voltage.
Most charge pumps comprise some variation of the basic charge pump 10 shown in the schematic diagram of FIG. 1. The basic charge pump 10 configuration includes a ring oscillator 12 which provides a square wave or pulse train having voltage swings typically between ground and the most positive external power supply voltage, Vcc. An inverter 14, buffer amplifier, or Schmnitt trigger circuit may be used to sharpen the edges of the oscillating output signal of the ring oscillator 12. When the ring oscillator 12 produces a voltage close to ground, the input to a capacitor 16 from inverter 14 is low. When the input to capacitor 16 is low, node 22 passes a charge of Vcc through diode 18 to node 26. At node 26 the received charge is approximately Vcc minus a threshold voltage, i.e. Vccxe2x88x92Vt (where xe2x80x9cVtxe2x80x9d is the threshold voltage). Since the input to capacitor 16 is low, capacitor 16 is pre-charged to the voltage Vccxe2x88x92Vt at node 26.
When the ring oscillator 12 produces a voltage close to Vcc, the input to capacitor 16 from inverter 14 is high. During this period, Vcc is supplied to the capacitor 16 and, together with the pre-charged value of Vccxe2x88x92Vt, passes a charge 2 Vccxe2x88x922 Vt to the load voltage terminal 24, Vccp. The additional Vt voltage drop is caused by diode 20. Vccp is the output voltage of charge pump 10. Capacitor 16 is prevented from discharging to node 22 by diode 18. Given an input voltage of Vcc, Vccp will typically result in twice the voltage of Vcc, minus the threshold voltages, 2 Vt.
In the charge pump 10, one pulse of current is delivered to the load voltage terminal 24 for every clock cycle of the ring oscillator 12, during the half of the clock cycle when the output of ring oscillator 12 is high. When the output of ring oscillator 12 is low, the other half of the clock cycle, capacitor 16 is pre-charged and voltage is not delivered to the load voltage terminal 24. These half clock cycles are commonly referred to as phases. Therefore, the charge pump 10 delivers a load voltage during a first phase and pre-charges capacitor 16 during a second phase. Although this second phase is necessary to pre-charge the capacitor 16, since no current is delivered to the load voltage terminal 24 during this second phase, it may be difficult to attain and maintain a final desired voltage, Vccp. Accordingly, charge pumps 10 have typically included two FIG. 1 circuits to operate out of phase with their outputs commonly connected to produce the load voltage at terminal 24 for each cycle of the ring oscillator 12 by utilizing both states of each ring oscillator cycle. This is known in the art as a two phase pump.
In most integrated electronic circuits, including memory chips, it is desirable that the final pump voltage at the load be reached as quickly as possible. Proper device functions and attributes, such as the integrity of stored data, cannot be guaranteed until the pump voltage has reached the proper value. However, the circuitry presently used for such a system is often inefficient in terms of size, power consumption and number of components. Therefore, there exist a need for a more efficient charge pump system.
The present invention relates to an improved charge pump system. Current charge pump systems contain a first boot circuit to provide a pump voltage Vccp as an output through an output transistor. Typically, such systems also contain a second boot circuit to provide a voltage greater than the pump voltage Vccp for driving the gate of the output transistor, to ensure that the pump voltage Vccp produced by the first boot circuit is passed to the drain of the output transistor and provided as output voltage Vccp. The present invention eliminates the need for the separate pre-charge capacitor and associated pre-charge circuitry found in current systems to pre-charge the capacitor of the second boot circuit, through the use of a single strategically placed diode. The net effect is a more efficient and smaller charge pump circuit.